1. Field of the invention
The present invention relates to novel coated substrates having an ultrathin, single-layer or multi-layer coating, in which coated substrates the coating material is a polyester, and also a process for the production of such coated substrates and novel polyesters as intermediate products for these coated substrates.
Coated substrates have a wide range of applications in industrial technology. For example, the frictional properties of materials can be tailored to a required purpose by modifying the surface. Furthermore, the coating may be a protective film for the substrate underneath, in order to preserve its special surface properties. In particular, however, coated substrates have recently been used as components in structural elements for optical communications technology and as electronic and optoelectronic information stores.
Particularly for the electronic and optical applications, it is necessary to produce ultrathin, defect-free multilayer coatings whose layers have a high degree of order and an adjustable layer thickness which is as homogeneous as possible, and this high degree of order should also be retained when a large number of coating layers are present.
The thin layers produced from the organic polymers form the basis of defect-free systems having order at the molecular level, as required, for example, for
optical applications (directional radiation with little damping, for example optical waveguides having nonlinear optical properties), PA1 electrical applications (electrical conductors of high anisotropy, for example one-dimensional or two-dimensional conductors in the field of molecular electronics), PA1 "host lattices" for defined incorporation or specific binding of functional groups or molecules (for example biomolecules for sensors). PA1 B is the skeleton of an aliphatic, araliphatic or aromatic dicarboxylic acid, PA1 n assumes values from 2 to 80 and at least one of the substituents R.sup.1 to R.sup.4 represents an aliphatic radical having an essentially normal chain and each of the other radicals denotes hydrogen or methyl, the aliphatic radical having at least 8 C atoms and it being possible for bonding of the aliphatic radicals having an essentially normal chain to the skeleton A or B also to take place via groups containing heteroatoms, and, where there are two aliphatic radicals having an essentially normal chain on one skeleton, these radicals are identical, and furthermore R.sup.1 and R.sup.2 together and R.sup.3 and R.sup.4 together may form an alkylene chain.
2. Description of the Related Art
It is already known that monomolecular layers can be produced from amphiphilic low molecular weight molecules, each having a polar or nonpolar end, such as, for example, long-chain fatty acids, on the surface of a non-solvent, such as water, in which monomolecular layers all polar ends are directed, for example, towards water, while the nonpolar ends project into the gas space above. For this purpose, small amounts of such amphiphilic substances are dissolved in a suitable solvent and, for example, introduced onto a water surface of sufficient size, where they spread, with evaporation of the solvent, to form an initially noncohesive, monomolecular layer. By moving a suitable barrier, the water surface is reduced in size and the resulting increase in the surface tension is measured as a function of the area still available (that is to say the surface pressure/area graphs which are known to those skilled in the art and are also known as .pi./A isotherms, are recorded).
This gives a cohesive, monomolecular layer exhibiting order at the molecular level. When this quasi-solid state of this monomolecular layer is reached, further movement of this barrier meets with a clearly measurable resistance, which indicates that this state has been reached. Suitable substrates can then be immersed through the monomolecular layer into the water and in turn become coated with a monomolecular layer of the amphiphilic substance during immersion and/or withdrawal, the monomolecular order in the individual layers being retained.
The stated process can be repeated. This coating technique is known to the skilled worker as the Langmuir-Blodgett technique.
However, coatings of monomeric amphiphilic substances are chemically, thermally and mechanically unstable, that is to say the molecular order may be lost through chemical and/or physical effects. Attempts have therefore been made to coat substrates with amphiphilic substances having a reactive double bond or multiple bond and subsequently to crosslink the said substances to give a polymer, for example by means of high-energy radiation.
Frequently investigated examples for the polymerization of unsaturated amphiphilic substances are diacetylenecarboxylic acids and .omega.-tricosenoic acid. The serious disadvantages of these substances are their high reactivity, with the result that they are often difficult to obtain in pure form. In the course of the polymerization, after transfer onto the substrate, shrinkage often results in defects, such as macroscopic cracks in the coating.
Attempts have also been made to include polymeric substances, such as polystyrene and polymethyl methacrylate, in the Langmuir-Blodgett technique. However, polymers give rise to difficulties in the alignment of the polar or the nonpolar structural components, owing on the one hand to the random disorder in the polymer chain and on the other hand to superstructures of the polymers. It is therefore always necessary to determine whether the amphiphilicity (that is to say the difference in polarity between the polar and the nonpolar groups) is sufficient to obtain coatings of high molecular order and good reproducibility. The so-called Y structure, in which the polar and the nonpolar structural elements are located opposite one another from one layer to the other in multilayer coatings, is frequently desirable here. For special applications, for example nonlinear optically active layers, this Y structure may be disadvantageous; with the polyesters described further below, it is also possible to produce layers of the X or Z type under suitable experimental conditions. In Journal of Molecular Electronics 1 (1985), 3-17, the subject matter discussed is described in detail.